At present, acidic chemical silver plating solution is commonly used for a silver-plated process in the chemical plating industry; the main problems brought about by using the acidic formula and its corresponding process are:
1. Gnawing and Corrosion of Copper Wires
At present, a nitric acid system is the most popularly system in the world used for the immersion silver. Nitric acid is a strong oxidizer, as well as a strong corrosive. It firstly corrodes parts with high stress in wires, thus causing stress corrosion, with the result that the wires become thin or are gnawed locally. Therefore, the immersion plating time must be strictly limited within 1 minute in a nitrate-based chemical silver plating process. Otherwise, a phenomenon that the wires become thin or become gnawed will appear. However, when the immersion plating time is only 1 minute, there is not enough time for reagents to get to the bottom of a blind hole which results in that the copper is exposed due to the bottom of the blind via not being plated by the silver.
2. Very High Carbon Content in the Silver Plating Layer
For delaying the corrosion of the nitric acid on the copper and promoting the reagents to get into the blind hole, a corrosion inhibitor that easily forms a protective membrane on copper and a penetrant with good permeability must be added into many nitrate-based chemical silver plating solutions. A corrosion inhibitor and penetrant can coprecipitate with silver, thus the silver layer so-called ‘organic silver’ is formed, that is, the silver layer contains plenty of organics, or the silver layer contains a high content of carbon. The content or purity of the silver of the ‘organic silver’ layer of these companies is only 70%, that is, the content of carbon is up to about 30%. High carbon content not only influences the conductivity of the silver layer, but also influences the weldability, corrosion resistance of the silver layer and increases high-frequency loss.
3. Void will be Formed in the Solder During Welding, thus Influences the Welding Strength
An acidic chemical silver layer contains a large amount of organics or carbon, which can react with the oxygen in the air during high temperature welding to produce carbon monoxide or carbon dioxide which can get into the melted solder. Because of the very high specific gravity of the melted solder, gas is hard to escape after getting into the solder, and finally has been condensed in the solder and then voids are formed. The thicker the silver plating layer is, the higher the total carbon content is, the more or larger the formed voids are, and most of which are centralized at the upside close to the welding pad.